To be able to compete with and replace fossil fuel energy carriers such as coal, oil and natural gas, lignocellulosic biomass would benefit from some form of pre-treatment method to overcome inherent drawbacks. The pre-treatment method torrefaction has been shown to improve biomass fuel qualities such as energy density, water content and milling, feeding and hydrophobic properties [1-4]. These improvements establish torrefaction as a key process in facilitating an expanding market for biomass raw materials. Torrefaction is a thermal pre-treatment method that normally takes place in an substantially inert (oxygen free) atmosphere at a temperature of about 220-600° C. During the process course a combustible gas comprising different organic compounds is produced from the biomass feedstock in addition to the torrefied biomass.
The process of producing a torrefied material from lignocellulosic biomass can be said to include four stages:    1) a drying step, wherein free water retained in the biomass is removed;    2) a heating step in which physically bound water is released and the temperature of the material is elevated to the desired torrefaction temperature;    3) a torrefaction stage, in which the material is actually torrified and which starts when the material temperature reaches about 220° C.-230° C. During this stage, the biomass partly decomposes and gives off different types of volatiles, such as hydroxy acetone, methanol, propanal, short carboxylic acids etc. In particular, the torrefaction stage is characterised by decomposition of hemicellulose at temperatures from 220° C.-230° C., and at higher torrefaction temperatures cellulose and lignin also starts to decompose and give off volatiles; cellulose decomposes at a temperature of 305-375° C. and lignin gradually decomposes over a temperature range of 250-500° C.;    4) a cooling step to terminate the process and facilitate handling. The torrefaction process is terminated as soon as the material is cooled below 220° C.-230° C.